Protein Synthesis NotesTwo stage processing of information from DNA to

proteins Gene Expression

Molecular Genetics Unit

https://www.youtube.com/watch?v=suN-sV0cT6c

Review

• Proteins are composed of amino acids.• Components: amino group, carboxyl

group, hydrogen, R group.• 20 different R groups/amino acids.

• Different proteins are made by combining these 20 amino acids in different combinations.

• Proteins are manufactured (made) by the ribosomes.

Genes• Chromosomes are divided into segments called genes.• Genes are directions for building all the proteins needed

by an organism.– Gene Protein Phenotype

Not all genes are active (expressed) at the same time.

Why? Because the cell would produce many molecules it did NOT need.• All cells in an organism have the entire genome.• All cells in an organism do NOT express the same genes.• Different kinds of cells require different kinds of molecules

to function.• Example: RBC’s need hemoglobin to bind O2, but other

cells do not.

Gene ExpressionGene expression (protein synthesis) is when the product of a gene (a specific protein) is being actively produced by a cell.• Some genes are rarely expressed… adrenaline.• Some genes are constantly expressed… hair growth. • Some genes are expressed for a time, then turned off (cyclical)…

estrogen.

Adrenaline: gene rarely “on.”

Estrogen: gene cyclically “on.”

When a gene is “on,” a protein will be synthesized.

Two stages of protein synthesis:

1. DNA is transcribed into mRNA: Transcription

2. mRNA is translated into a protein: Translation

The “Central Dogma”• Flow of genetic information in a cell.• How do we move information from DNA to

proteins?transcription

translation

replication

protein

RNA

DNA

trait

Transcription

Definition: segments of DNA (gene) serve as templates to produce complementary RNA molecules.Why does the information have to be transferred? • DNA has code for -- protein that needs to be made.• Proteins are made -- in the ribosomes (in the cytoplasm).• DNA is too large -- to leave the nucleus.• RNA can -- leave the nucleus.

The enzyme RNA polymerase binds to DNA, unzips DNA, and uses DNA as a template to build a single stranded mRNA.• Creates a strand of mRNA with DNA code on it.• Remember, uracil replaces thymine in RNA.• Only one strand of DNA is used as a template and

transcribed.• mRNA is created in 5’3’ direction.

How does RNA polymerase know where to start and stop making a strand of RNA?• RNA polymerase can only bind to promoters, regions of

DNA that have specific base sequences.– Promoters act as signals for RNA polymerase.

• Terminators in DNA sequence causes transcription to stop.– Occur at the end of the gene.– Causes RNA polymerase to dissociate from DNA.

RNA Editing• Gene is composed of exons and

introns.– Exons code for protein.– Introns do not code for protein.

• Both introns and exons are transcribed by DNA polymerase to make pre-mRNA.

• But… introns are not allowed to leave the nucleus.– Introns are removed from pre-mRNA.– Exons are then sliced together to make

mature mRNA.

Think-Pair-Share1. For the following DNA sequence, write the mRNA molecule

that would be created.ATGCGGTTAGGCTAGTAC

2. Fill in the following table about transcription.Transcription

Where does the process occur?What enzyme is used during the process?What is the template that is read?What is the start signal?What polymer is formed?What direction is the new polymer formed?What is the stop signal?

After transcription, mRNA is small enough to get through the nuclear envelope pores – mRNA leave nucleus.– mRNA travels to a ribosome (may be free or bound).

Nuclear pore.

mRNA exits the nuclear pore after RNA editing. mRNA small enough to leave the nucleus.

The Genetic Code

• Remember, 20 amino acids are commonly found in polypeptides (proteins).– The order in which amino acids are joined

determines the shape/function of a protein.

• How can the RNA code (with only four letters) carry instructions for 20 different amino acids?– The genetic code is read three “letters” at a

time, so that each “word” is three bases long and corresponds to a single amino acid.

– Each three-letter “word” in mRNA is known as a codon.

How to Read Codons

• There are 64 possible codons.

• Most amino acids can be specified by more than one codon.

Start and Stop Codons• Start codon: AUG (also codes for methionine).• Stop codons: UAA, UAG, UGA.– End translation.– Polypeptide is complete.

Think-Pair-ShareFind the amino acid sequence coded by the following mRNA strand: CACCCAUGGUGA

Think-Pair-ShareFind the amino acid sequence coded by the following DNA strand: TACTTGCTGATT

Translation1. Begins are AUG, the start codon.

a. Each tRNA has an anticodon whose bases are complementary to the bases of the codon.

b. The ribosome positions the start codon to attract its anticodon.c. Ribosome also binds the next codon and its anticodon.

2. The ribosome joins the first two amino acids, and breaks the bonds between methionine (first amino acid) and its tRNA.

3. Empty tRNA floats away.

4. Ribosome binds another tRNA.5. The ribosome moves along the

mRNA binding new tRNA molecules and amino acids.

6. The process continues until the ribosome reaches one of the three stop codons.

a. Polypeptide is complete.b. mRNA and protein are released

from the ribosome.

https://www.youtube.com/watch?v=2zAGAmTkZNY